In order to transfer oil and gas fluids from a storage tank on the seabed to a transport tanker, it is known to use a loading riser supported by a tension buoy anchored to the seabed by a rigid main riser. A pick up line from the end of the loading riser is connected to a marker buoy which floats on the surface of the sea. The tension buoy and loading riser are maintained below the sea surface. Normally, only the marker buoy is evident on the surface of the sea. However, problems of entanglement between the rigid main riser and the loading riser have been experienced as the tanker hauls in the marker buoy to draw up the loading riser.
One example of such a system is described below in detail with reference to FIGS. 1 and 2.
A prior art system for transferring load from a storage tank 1 (located as a separate unit sub sea or integrated into a production platform/vessel) to a tanker 2 on the sea surface is illustrated in FIGS. 1 and 2. Here, the load is a hydrocarbon fluid comprising oil, gas and/or other components from the storage tank 1, flowing to a riser base 3 via the flow line 1 a and further up a main riser 4, through a tension buoy 14 and further through an active transfer device (collectively 13 and 20)
The active transfer device located on top of the tension buoy 14 comprises a swivel 20 and a gooseneck 13 leading up from a loading riser 10. The loading riser 10 may be a flexible riser or hose, which hangs freely down from the swivel/gooseneck on the tension buoy 14 when not in use (i.e. on standby) as shown in FIG. 2. When in use, the loading riser 10 is connected to the tanker 2, and the swivel gooseneck 13 is free to swivel in order to keep the tanker 2 downwind from the tension buoy 14 as the wind shifts direction during loading as illustrated in FIG. 1.
In order to transfer the loading riser 10 from the standby mode (hanging down), to the loading mode (when it is connected to the tanker 2), a pick up system is required. A pick up system can be implemented in many ways, but the pick up system associated with the present invention may comprise the following known elements: A pick up line 11 runs from the free end of the loading riser 10 (the end valve) to a spring buoy 15 near the level of the tension buoy 14. An intermediate pick up line 11a runs to a master link 16 some 15 m below the sea surface 5. An upper pick up line 11b runs from the master link 16 to a marker buoy 17 at the sea surface. The present invention may also work with other configurations of a pick up system. Hence the pick up system is not part of the invention.
During connection the tanker moves close to the marker buoy 17, picks it up and connects it to the tanker's pull in line 18 and so to the master link 16. The tanker's pull in line 18 is connected to a winch which is a part of the tanker's tension buoy 19. The winch is activated and pulls the pull in line 18 and the pick up line 11a in towards the tension buoy 19. The loading riser 10 end valve follows the pick up line and engages in the tension buoy 19 when the pick up line is pulled in to its end. Transfer of the hydrocarbons can now start.
After transfer of the load to the tanker, the loading riser 10 and the pick up system 11 needs to be disconnected. The tanker's pull in line 18 is paid out until the master link 16 appears in the connection device on the tanker's tension buoy 19. At this point the loading riser 10 is again in a vertical position. The upper pick up line 11b and the marker buoy 17 are then connected to the master link 16 and the pull in line 18 is disconnected. The upper pick up in line 11b and the marker buoy 17 are again an integral part of the pick up system 11. They are then thrown overboard and the tanker leaves the site. The system is now again in the standby mode.
The system described above is a very simple pickup system. It works well during loading and disconnection operations, but it runs a significant risk of becoming entangled in the swivel/gooseneck details 20 and 13 as well as wrapping itself around the rigid riser 4 during standby operation. If this occurs, the pick up process will become impossible without the aid of a remotely controlled vehicle to disentangle the system. This is why the industry has in the past employed much more elaborate and expensive pick up systems, where this particular risk is eliminated. Such systems become exceedingly expensive as well as cumbersome from an operational point of view in deep water, and finally at some depth they are no longer a feasible alternative.
An objective of the present invention is to prevent the pickup line from becoming entangled as well as wrapped around the rigid riser. The invention enables the use of a simple pick up system, such as that described above. As a result water depth is no longer a critical parameter for loading systems like the one described above (or for similar systems).